1. Field of the Invention
The present invention relates to a temperature compensated crystal oscillator for use in cellular phones and other small-scale electronic devices.
2. Description of the Related Art
Over the past several years, the specifications of temperature compensated crystal oscillators which cellular phone makers demand of crystal oscillator makers have thoroughly changed.
In the past, in order to secure as many telephone lines as possible in a limited frequency band, it was common to compress the frequency band width of each line. Accordingly, it became necessary to make the temperature compensated crystal oscillator frequencies as precise as possible.
However, due to the subsequent enlargement of the frequency band used by cellular phones, the demand switched from precision of frequencies to a demand for low cost light weight temperature compensated crystal oscillator.
In regards to smallness and lightness, products of various crystal oscillator makers are currently roughly the same. This arises from the development of temperature compensated crystal oscillators having the same size in view of mutual compatibility, which therefore results in all such products having substantially the same weight.
As a result of substantially the same performance and dimensions of temperature compensated crystal oscillators, the recent main concern of cellular phone makers with respect to the temperature compensated crystal oscillators is cost.
Thus, in order to maintain price competitiveness and stay ahead in the marketplace, crystal oscillator makers are making every effort to reduce the cost of temperature compensated crystal oscillators.
Incidentally, the temperature compensated crystal oscillator uses as its oscillation source a 10 MHz range AT-cut crystal resonator (crystal unit), which cooperates with any frequency varying means to constitute a temperature compensating circuit thereby canceling the temperature characteristics depend on cubic finction of the AT-cut crystal resonator, to consequently stabilize the oscillating frequency. Depending on the configuration of the temperature compensating circuit, it is roughly classified into an analog temperature compensated crystal oscillator and a digital temperature compensated crystal oscillator.
Furthermore, in recent years, in stead of combining parts, a one chip semiconductor integrated circuit has been used to make an analog temperature compensation crystal oscillator (hereinafter referred to as a one chip analog temperature compensated crystal oscillator), which is disclosed in Kuichi Kubo etc., "1996 IEEE INTERNATIONAL FREQUENCY CONTROL SYMPOSIUM" pp 728-734.
In this publication, the composition of the one chip analog temperature compensated crystal oscillator and the method of adjusting the oscillating frequency basically resembles that for the digital temperature compensated crystal oscillator, with the result that their methods of cost reduction also resemble each other and lowering the cost of adjustment has become a crucial point.
Because analog temperature compensated crystal oscillators compensate temperature by combining electronic parts that have different temperature characteristics, finding the optimum combination of parts and raising the percentage of obtaining passed products without changing the parts, has become a main concern.
Alternatively, another proposal is also disclosed in Japanese Laid-open Patent Pub. No. 6-85538 in which a gradient correction capacitor is used for the analog temperature compensated crystal oscillator in order to secure good frequency precision even in the case of use of inexpensive electronic parts.
In any case, the makers of temperature compensated crystal oscillators are pursuing efforts to reduce the costs of temperature compensated crystal oscillators.
However, temperature compensated crystal oscillators having conventional configurations contain an intrinsic barrier to any real cost reduction.
The analog temperature compensated crystal oscillators involve large parts costs and mounting costs due to too many constituent electronic parts. As well, the cost of refining and readjusting substandard products, found in the inspections at shipment, into passed products, is extremely high.
This is also true for the analog temperature compensated crystal oscillators using the gradient correction capacitor.
In the case of the digital temperature compensated crystal oscillators, it is difficult to achieve reduction of costs due to lack of progress in reducing the costs incurred to write the data for temperature compensation into memory. In addition, since the integrating circuit used to suppress FM noise can not be eliminated, reducing the number of parts is also posing a significant problem.
Although the one chip analog compensated crystal oscillator appears to be superior to the digital temperature compensated crystal oscillator in that the integrating circuit is not necessary, the process of writing of data is nearly the same as that of the digital temperature compensated crystal oscillator. Therefore, the task of lowering this cost is not progressing well. In summary, with the current construction of the conventional temperature compensated crystal oscillator, it is extremely difficult to achieve a substantial reduction in costs.
The present invention was therefore conceived in order to solve the above problems with its object to reduce the costs of the temperature compensated crystal oscillators.